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The dorsal stream (green) and ventral stream (purple) are shown. They originate from a common source in visual cortex. The dorsal stream is responsible for detection of location and motion.

Motion perception is an aspect of spatial perception and is the process of inferring the speed and direction of elements in a scene based on visual input. Although this process appears straightforward to most observers, it has proven to be a difficult problem from a computational perspective, and extraordinarily difficult to explain in terms of neural processing.

The observer's visual input is generally insufficient to uniquely determine the 'true' velocity in a visual scene. In monocular vision for example, the visual input will be a 2D projection of a 3D scene. The motion cues present in the 2D projection will by default be insufficient to reconstruct the motion present in the 3D scene. Put differently, many 3D scenes will be compatible with a single 2D projection. The problem of motion estimation generalizes to binocular vision when we consider occlusion or motion perception at relatively large distances, where binocular disparity is a poor cue to depth.

These issues become more apparent when we look at visual illusions involving motion. A well known example is the barberpole illusion. When a diagonally-striped pole is rotated around its longer axis, so that the stripes are moving in the direction of the pole's shorter axis, it nonetheless appears the stripes are moving in the direction of its longer axis.

The barberpole illusion. The stripes on the pole appear to move downward along the pole's longer axis. However, the actual motion is a rotation of the barberpole around its longer axis, which should have created a rightward motion.

The aperture problem. A neuron in visual cortex has a limited receptive field. It will only 'see' part of the barberpole shown above. Though the motion of the barberpole in the top figure appears to be downward, the motion apears to be both downward and rightward when viewed through an aperture in the bottom figure. In reality the motion is identical in both figures.

In addition to the problems of motion perception mentioned above, a number of issues arise due to the physiology of the brain. Each neuron in the visual system is sensitive to visual input in a small part of our visual field, as if each neuron is looking at the visual input through a small apperture. At the resolution of this apperture visual cues can often be approximated by straight lines. The motion direction of a straight line is fundamentally ambiguous, because the motion component parallel to the line cannot be inferred based on the visual input.

In cases where motion cannot be determined based on visual input alone, the visual system is thought to rely on prior assumptions. In the second figure the visual input and prior assumptions together make it appear the stripes are moving to the bottom-right.

Individual neurons initially estimate motion locally within their receptive field. Because each neuron will suffer from the aperture problem the estimates from many neurons are then integrated into a global motion estimate. This appears to occur in Area MT/V5 in human visual cortex.

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Neuropsychology

Area V5 seems to be important to the processing of visual motion and damage to this area can disrupt motion perception. Neuropsychological studies of a patient who could not see motion, seeing the world in a series of static "frames" instead, suggested that visual area V5 in the human is homologous to area MT in the primate. [1][2]

First-order motion perception

First-order motion perception refers to the perception of the motion of an object that differs in luminance from its background, such as a black bug crawling across a white page. This sort of motion can be detected by a relatively simple motion sensor designed to detect a change in luminance at one point on the retina and correlate it with a change in luminance at a neighbouring point on the retina after a delay. Sensors that work this way have been referred to as Reichardt detectors (after the scientist W. Reichardt, who first modelled them), [3] motion-energy sensors, [4] or Elaborated Reichardt Detectors. [5] These sensors detect motion by spatio-temporal correlation and are plausible models for how the visual system may detect motion. Debate still rages about the exact nature of this process. First-order motion sensors suffer from the aperture problem, which means that they can detect motion only perpendicular to the orientation of the contour that is moving. Further processing is required to disambiguate true global motion direction.

Second-order motion perception

Second-order motion is motion in which the moving contour is defined by contrast, texture, flicker or some other quality that does not result in an increase in luminance or motion energy in the Fourier spectrum of the stimulus.[6][7] There is much evidence to suggest that early processing of first- and second-order motion is carried out by separate pathways.[8] Second-order mechanisms have poorer temporal resolution and are low-pass in terms of the range of spatial frequencies that they respond to. Second-order motion produces a weaker motion aftereffect unless tested with dynamically flickering stimuli.[9] First and second-order signals appear to be fully combined at the level of Area V5/MT of the visual system.

Motion integration

Having extracted motion signals (first- or second-order) from the retinal image, the visual system must integrate those individual local motion signals at various parts of the visual field into a 2-dimensional or global representation of moving objects and surfaces.

The aperture problem

The Aperture Problem. The grating appears to be moving down and to the right, perpendicular to the orientation of the bars. But it could be moving in many other directions, such as only down, or only to the right. It is impossible to tell unless the ends of the bars become visible in the aperture.

Each neuron in the visual system is sensitive to visual input in a small part of the visual field, as if each neuron is looking at the visual field through a small window or aperture. The motion direction of a contour is ambiguous, because the motion component parallel to the line cannot be inferred based on the visual input. This means that a variety of contours of different orientations moving at different speeds can cause identical responses in a motion sensitive neuron in the visual system.

Individual neurons early in the visual system (LGN or V1) respond to motion that occurs locally within their receptive field. Because each local motion-detecting neuron will suffer from the aperture problem the estimates from many neurons need to be integrated into a global motion estimate. This appears to occur in Area MT/V5 in human visual cortex.

Motion in depth

As in other aspects of vision, the observer's visual input is generally insufficient to determine the true nature of stimulus sources, in this case their velocity in the real world. In monocular vision for example, the visual input will be a 2D projection of a 3D scene. The motion cues present in the 2D projection will by default be insufficient to reconstruct the motion present in the 3D scene. Put differently, many 3D scenes will be compatible with a single 2D projection. The problem of motion estimation generalizes to binocular vision when we consider occlusion or motion perception at relatively large distances, where binocular disparity is a poor cue to depth. This fundamental difficulty is referred to as the inverse problem.